There is a need for small presses to cut and mold miniature components. One type of large press includes a motor-driven pump that pumps hydraulic fluid to a high pressure, and valves that direct the fluid into chambers at a push rod to push it down and then push it up again. The hydraulic fluid reservoir, pump and motor for driving it, and valves for controlling movement of the push rod, are of large size and considerable cost, and are unsuitable for miniature presses. A relatively simple type of miniature press includes a crank mechanism for moving down a push rod, with the crank mechanism operated by a small electric motor or even by hand. It is difficult to closely control movement of the crank-driven push rod, with only a sinusoidal force-displacement profile usually present. A miniature press, such as one that applies a force of no more than several (seven) tons to the push rod, which was of simple and low cost construction and yet which could be precisely controlled, would be of value.
In accordance with one embodiment of the present invention, a miniature press is provided for cutting and molding miniature components by moving down a push rod, wherein downward force on the push rod is obtained by a piezoelectric actuator that is coupled through a hydraulic transmission that amplifies movement of the piezoelectric actuator. The piezoelectric actuator, or piezoactor, is connected to a pressure piston that moves in a pressure cylinder. When the piezoactor moves the pressure piston toward a first end of the pressure cylinder, the piston compresses hydraulic fluid. The hydraulic fluid is coupled to a rod chamber that contains hydraulic fluid that presses against an upwardly-facing shoulder on the push rod to move down the push rod. The area of the push rod shoulder that is exposed to hydraulic fluid is a small fraction of the area of the pressure piston that pushes against hydraulic fluid in an end of the pressure cylinder. As a result, small movement of the piezoactor is converted into large movement of the push rod. The piezoactor responds almost instantaneously to changes in electricity applied to it, so close control of go push rod movement is achieved.
A force sensor that senses force applied by the push rod and a movement sensor that senses movement and/or position of the push rod, are connected to a control that delivers current to the piezoactor, to closely control movement of the piezoactor and therefore of the push rod. The very low moving mass of the piezoactor and hydraulic fluid increases control of movement of the piezoactor and of the push rod. The hydraulic transmission avoids the need to move the larger mass of a mechanical connection and avoids the xe2x80x9cplayxe2x80x9d in mechanical parts that would decrease control;
The pressure cylinder preferably has first and second opposite ends, with hydraulic fluid in each end. Also, the rod chamber preferably has a second chamber portion that opens to a downwardly-facing shoulder of the push rod. The piezoactuator can be moved in a second direction that is opposite to the first, to move the pressure piston so as to pressurize fluid in the second end of the pressure cylinder and thereby push up the push rod. By close control of upward movement of the push rod as well as downward movement, efficient operation of the press can be obtained.
The press can include a plurality of piezoactors that are energized in unison to move separate pressure pistons whose ends are connected to the same chamber portions that move the push rod. This allows for large displacement distances of the push rod, to adapt the press to different product settings.
The provision of a force sensor that senses force on the push rod and a rod position sensor, makes it possible to closely monitor operation of the press. In one example, an increase in force required for a given push rod movement, may indicate that the tool is worn and needs replacing. Such sensing enables close control of push rod movement, which can be useful to enable the processing of different materials or materials of different thicknesses, using the same tooling.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.